Disk drive having position sensor for ramp load and unload, and method for its use

ABSTRACT

Disclosed is a disk drive having at least one disk for storing data. The disk drive includes at least one transducer for reading or writing data to or from the disk. The transducer is attached to an actuator which positions the transducer with respect to the disk. The actuator includes a controllable motor which is used to move the actuator and the transducer attached thereto. The disk drive also includes a ramp for off loading the transducer or for parking the transducer off of the surface of the disk. Also disclosed is apparatus and methods for measuring and controlling the movement of the actuator and attached transducer while the transducer is coming from a position off of the ramp and onto the surface of the disk.

This application is a continuation of application Ser. No. 08/483,040,filed Jun. 7, 1995 now abandoned which is a divisional of applicationSer. No. 08/252,667 filed on Jun. 2, 1994, now U.S. Pat. No. 5,455,723.

FIELD OF THE INVENTION

The present invention pertains to the field of disk drives which arealso called direct access storage devices (DASD).

More particularly, this invention pertains to a ramp used to load andunload a transducing head onto and off of a disk, and for a method ofcontrolling the movement of the transducing head as it passes over theramp.

BACKGROUND OF THE INVENTION

One of the key components of a computer system is a place to store data.Typically computer systems employ a number of storage means to storedata for use by a typical computer system. One of the places where acomputer can store data is in a disk drive which is also called a directaccess storage device.

A disk drive or direct access storage device includes several diskswhich look similar to 45 rpm records used on a record player or compactdisks which are used in a CD player. The disks are stacked on a spindle,much like several 45 rpm records awaiting to be played. In a disk drive,however, the disks are mounted to spindle and spaced apart so that theseparate disks do not touch each other.

The surface of each disk is uniform in appearance. However, inactuality, each of the surfaces is divided into portions where data isstored. There are a number of tracks situated in concentric circles likerings on a tree. Compact disks have tracks as do the disks in a diskdrive. The tracks in either the disk drive or the compact diskessentially replace the grooves in a 45 rpm record. Each track in a diskdrive is further subdivided into a number of sectors which isessentially just one piece of the track.

Disks in a disk drive are made of a variety of materials. Most commonly,the disks used in rotating magnetic systems is made of a substrate ofmetal, ceramic, glass or plastic with a very thin magnetizable layer oneither side of the substrate. Such a disk is used in magnetic, andmagneto-optical storage. Storage of data on a such a disk entailsmagnetizing portions of the disk in a pattern which represents the data.Other disks, such as those used in CD's, are plastic. Data, such assongs, is stored using a laser to place pits in the media. A laser isused to read the data from the disk.

As mentioned above, to store data on a disk used in a rotating magneticsystem, the disk is magnetized. In order to magnetize the surface of adisk, a small ceramic block known as a slider which contains at leastone magnetic transducer known as a read/write head is passed over thesurface of the disk. Some ceramic blocks contain a separate read headand a separate write head. The separate read head can be amagnetoresistive head which is also known as an MR head. The ceramicblock is flown at a height of approximately six millionths of an inch orless from the surface of the disk and is flown over the track as thetransducing head is energized to various states causing the track belowto be magnetized to represent the data to be stored. Some systems nowalso use near contact recording where the slider essentially rides on alayer of liquid lubricant which is on the surface of the disk. With nearcontact recording, the ceramic block passes even closer to the disk.

To retrieve data stored on a magnetic disk, the ceramic block or slidercontaining the transducing head is passed over the disk. The magnetizedportions of the disk generate a signal in the transducer or read head.By looking at output from the transducer or read head, the data can bereconstructed and then used by the computer system.

Like a record, both sides of a disk are generally used to store data orother information necessary for the operation of the disk drive. Sincethe disks are held in a stack and are spaced apart from one another,both the top and the bottom surface of each disk in the stack of diskshas its own slider and transducing head. This arrangement is comparableto having a stereo that could be ready to play both sides of a record atanytime. Each side would have a stylus which played the particular sideof the record.

Disk drives also have something that compares to the tone arm of astereo record player. The tone arm of a disk drive, termed an actuatorarm, holds all the sliders and their associated transducing heads, onehead for each surface of each disk supported in a structure that lookslike a comb at one end. The structure is also commonly called an Eblock. A portion of metal, known as a suspension, connects the slidersto the E block. At the other end of the actuator is a coil which makesup a portion of an voice coil motor used to move the actuator. Theentire assembly is commonly referred to as an actuator assembly.

Like a tone arm, the actuator arms rotate so that the transducers withinthe sliders, which are attached to the actuator arm can be moved tolocations over various tracks on the disk. In this way, the transducingheads can be used to magnetize the surface of the disk in a patternrepresenting the data at one of several track locations or used todetect the magnetized pattern on one of the tracks of a disk. Actuatorssuch as the ones described above are common to any type of disk drivewhether its magnetic, magneto-optical or optical.

One of the most critical times during the operation of a disk drive isjust before the disk drive shuts down. When shutting down a disk drive,several steps are taken to help insure that the data on the disk ispreserved. In general, the actuator assembly is moved so that thetransducers do not land on the portion of the disk that contains data.How this is actually accomplished depends on the design of the drive.The disk drive design of interest for this invention includes a ramp.Disk drives with ramps are well known in the art. U.S. Pat. No.4,933,785 issued to Morehouse et al. is one such design. Other diskdrive designs having ramps therein are shown in U.S. Pat. No. 5,235,482and U.S. Pat. No. 5,034,837.

Typically, most of the ramp is situated off to the side of the disk. Aportion of the ramp is positioned over the disk itself. In operation,before power is actually shut off, the actuator assembly swings thesuspension or another portion of the actuator assembly up the ramp to apark position at the top of the ramp. This is much like a child runningup a playground slide backwards and sitting at the top of the slide.When the actuator assembly is moved to a position where parts of theassembly are at the top of the ramp, the sliders or ceramic blocks,which include the transducers, are positioned so that they do notcontact the disk. Commonly, this procedure is known as unloading theheads. Unloading the heads helps to insure that data on the disk ispreserved since, at times, unwanted contact between the slider and thedisk results in data loss on the disk.

Startup of a disk drive with a ramp is an even more critical time.Startup includes moving the actuator assembly so that the suspensionslides down the ramp and so that the slider flies when it gets to thebottom of the ramp. This is much like a water slide where the bottom ofthe pool is the disk and the slide is the ramp. When the rider gets tothe bottom, he "flies" by skimming across the water rather than touchingthe bottom of the pool. In other words, the best ramp control designsprevent contact between the slider and the disk so as to prevent anytype of data loss.

The most common mechanical design which assures that the slider will flyrequires a ramp with a very gentle slope. There are problems associatedwith this design. Most importantly, a gentle sloping ramp is longer thana short ramp and requires more space. Space is becoming more precious asthe form factor of the disk drive shrinks. Currently, the smallest diskdrive on the market has a disk with a diameter of 1.3". Also on themarket are PCMCIA form factor disk drives. The PCMCIA disk drives havethe length and width of a credit card. The height of these drivesvaries. The disk in such a drive has a diameter of about 1.8". Packing along ramp in such a small packages is difficult. Even if it can be done,there will be a push toward steeper ramps since with a steeper slopingramp, more of the disk surface can be devoted to storing data to satisfythe consumer's thirst for increased data capacity.

A way to accommodate a steeper ramp is to control the velocity of theslider as it moves down the ramp. If the velocity can be controlled, thedownward portion of the speed can be controlled so that the slider willnot result in the slider hitting the disk. U.S. Pat. No. 4,864,437issued to Couse et al. teaches one way of controlling the velocity ofthe slider as it moves down a ramp. In Couse et al., the voltage acrossa voice coil motor is monitored and controlled. The voltage across thevoice coil motor includes a small component of the total voltage knownas Back EMF. A voice coil motor includes magnets and an actuator coil.When the actuator coil cuts a magnetic field, Back EMF is generated. TheBack EMF varies as a function of the velocity of the actuator coilthrough the magnetic field produced by the magnets of the voice coilmotor and, presumably, as a function of the velocity of the actuatordown the ramp. Thus, it is possible to get an estimate of the rotationalvelocity of the actuator from the Back EMF of the actuator motor. Fromthe rotational velocity estimate and knowing the designed slope of theramp one can calculate the component of velocity in the verticaldirection (perpendicular to the surface of the disk). It is important tocarefully control the vertical velocity in order to prevent any damageto the disk surface.

The design of the velocity control in Couse et al. also has problems.Most importantly, the Back EMF is a very small component of the totalvoltage across the coil of the actuator. This component will also becomesmaller as additional current is passed through the coil. The Back EMFsignal is also prone to noise. In short, since the Back EMF component ofthe voltage across the actuator is small and prone to noise, it does notalways reliably reflect the actual velocity of the slider. In addition,as the operating temperature of the disk drive increases, the noiselevel increases making the Back EMF an even smaller component and evenmore prone to noise. If there happens to be an error indicating that thevelocity is slower than it actually is, then an increase in the actuatorcoil current may cause the velocity of the slider down the ramp toincrease to the point where the slider will contact the surface of thedisk. This could cause a head crash resulting in a loss of data. Itshould be remembered that contact between the disk and the slider mightnot instantly cause data loss. Many times it causes particle generationwithin the disk enclosure. Generated particles, although seemingly smallin everyday terms, are "boulders" to a slider that is flying at lessthan six millionths of an inch from the surface of the disk.

Another problem is that at the smaller form factors the torque constantof the actuator moves down drastically which means that the actuatormotor can not get the slider moving as quickly in a smaller form factordrive. For example, the torque constant in the current 1.8" PCMCIA typedrives is approximately 10-15% of the torque constant in a 2.5" drive(the next larger form factor). In essence, smaller form factor drivesmust use smaller actuator motors which produce less torque. This problemalso gets worse as the height dimension of the drive shrinks since asmaller actuator motor is used. Thus, as the actuator motors get smallerthere is less torque to get the actuator moving and therefore less BackEMF signal produced over the short amount of stroke for the sliders tomove down the ramp. The result is that the noise further drowns out thesmaller Back EMF signal produced by the slower moving actuator.

Yet another problem with using the Back EMF of the actuator motor todetermine speed is that the Back EMF varies with the temperature of thepermanent magnets and the coil resistance in the voice coil motor. Insmall form factor drives that may go into a laptop or sub-laptopcomputer there may be instances where the drive may park the sliders onthe ramp and, within minutes, slide them back down the ramp again. Inthese applications, the disk drive will be at or near the operatingtemperature of the drives. The operating temperature of a drive can beup to 50 degrees Celsius higher than the drive when it first starts fromroom temperature. The Back EMF can vary as much as 10-15% over such atemperature range. Of course this amount of difference in the Back EMFsignal translates into a 10-15% difference in the vertical component ofvelocity of the transducer which may well result in the head contactingthe disk.

Of course, the system can be designed to accommodate "worst case"situations but this results in a sub-optimal design. In addition, theprior art teaches no way of estimating what the initial values ofseveral data points might be so that the velocity function can beaccurately estimated while the transducer is on the ramp.

Although the measure of Back EMF is a closed loop process in terms ofvelocity control, the use of Back EMF does not indicate position. Thereis a problem, potentially, in not knowing the position of the transduceras it moves over the ramp.

Thus, what is needed is a device that can accurately and repeatablydetermine the velocity of the slider as it moves down the ramp withoutregard to temperature fluctuations, differing noise levels, or changesin the temperature of the actuator motor. In addition, what is needed isa device that allows an estimate of the velocity to be made from aconstant generated while the transducer is still parked on the rampmaking a single measurement. Preferably, the velocity of the slider willnot be determined based on the Back EMF of the actuator coil of theactuator assembly.

SUMMARY OF THE INVENTION

The invention is an apparatus and method for estimating the position ofthe head or transducer as it moves up or down the ramp. A reliablevelocity estimation is also made. The Back EMF of the coil is not usedwhich eliminates the problems associated with noise and the fact that asthe coils get smaller and smaller the Back EMF component also getssmaller.

In one set of preferred embodiments, one part of a two part sensor isattached to the actuator and another part to the housing of the diskdrive storage device. The output of the sensor indicates the particulardistance along the surface of the ramp. The sensor in this preferredembodiment is a Hall effect sensor and a button magnet. The outputvoltage of a Hall effect sensor varies in response to temperature.Disclosed is a method for making an initial measure while the transduceris atop the ramp and using it to estimate the output voltage for thevarious positions of transducer. A method of exactly calibrating thevelocity estimation using the track spacing on the disk as a gauge isalso disclosed.

Another preferred embodiment in the first set uses a capacitance probe.In each of these embodiments, the servo electronics have two parts. Onepart is for servoing while the transducer is positioned over the disk.Another servo electronics part is for servoing while the transducer ispositioned over the surface of the ramp. In each case a controller isused to switch between the servo electronic parts.

Another set of preferred embodiments does not use a separate two partsensor. Rather, the recording transducer is used and a portion of theramp is either magnetized or produces a magnetic field. The same servoelectronics can be used while the transducer is passing over the rampand while the transducer is passing over the disk.

All of the preferred embodiments are used ultimately to control themovement of the actuator which controls the velocity of the transducerup or down the ramp.

Advantageously, in each of these embodiments, the velocity of thetransducer as it travels down the ramp can be determined without concernregarding the noise level in the coil of a motor driving the actuator orregarding the size of the coil of the motor. In addition, thetemperature of the disk drive will have either no effect or the effectcan be corrected.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the present invention, reference can bemade to the accompanying drawings, in which:

FIG. 1 is an exploded view of a disk drive.

FIG. 2 is a isometric view of a ramp from a disk drive.

FIG. 3 is a isometric view of a load beam from a disk drive.

FIG. 4 is an schematic view of one preferred embodiment of the inventivevelocity controller.

FIG. 5 is a graph showing how the curve of voltage vs. distance from aHall effect sensor changes with respect to temperature.

FIG. 6 is a graph illustrating that, due to the short stroke of thevoice coil motor, only a small portion of the curve of voltage vs.distance from a Hall effect sensor is used in detecting distances inbetween the actuator and the small button magnet.

FIG. 7 is a graph showing how the curve of voltage vs. distance from aHall effect sensor changes with respect to distance.

FIG. 8A is a flow diagram showing a method for initially starting thedisk drive.

FIG. 8B is a flow diagram showing a method for starting the disk drive.

FIG. 9 is an schematic view of a second preferred embodiment of theinventive velocity controller. (optical)

FIG. 10 is an schematic view of third preferred embodiment of theinventive velocity controller. (capacitance probe)

FIG. 11 is an schematic view of fourth preferred embodiment of theinventive velocity controller. (magnetic strip)

FIG. 12 is a flow diagram showing an embodiment with a current carryingwire on a ledge of the ramp and a related electronic circuit for such asystem.

FIG. 13 is a top view of a portion of flex cable used in FIG. 12 thatwould be placed on the ledge of the ramp.

FIG. 14A is a side view of an actuator assembly having a capacitiveplate positioned between the arms of the E block.

FIG. 14B is a top view of FIG. 14A and includes several other portionsof a disk drive.

FIG. 15 is a schematic diagram associated with the capacitive sensorshown in FIGS. 14A and 14B.

These drawings are not intended as a definition of the invention but areprovided solely for the purpose of illustrating the preferredembodiments of the invention described below.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The invention described in this application is useful with allmechanical configurations of disk drives or direct access storagedevices ("DASD") having either rotary or linear actuation. FIG. 1 is anexploded view of a disk drive 10 having a rotary actuator. The diskdrive 10 includes a housing 12, and a housing cover 14. The housing 12and housing cover 14 form a disk enclosure. Rotatably attached to thehousing 12 on an actuator shaft 18 is an actuator assembly 20. Theactuator assembly 20 includes a comb-like structure 22 having aplurality of arms 23. Attached to the separate arms 23 on the comb 22,are load beams or load springs 24. Attached at the end of each loadspring 24 is a slider 26 which carries a magnetic transducer 50 (shownin FIG. 3). The slider 26 with the transducer 50 form what is many timescalled the head. It should be noted that many sliders have onetransducer 50 and that is what is shown in the Figures. It should alsobe noted that this invention is equally applicable to sliders havingmore than one transducer, such as what is referred to as an MR ormagneto resistive head in which one transducer 50 is generally used forreading and another is generally used for writing. On the end of theactuator arm assembly 20 opposite the load springs 24 and the sliders 26is a voice coil 28.

Attached within the housing 12 is a pair of magnets 30. The pair ofmagnets 30 and the voice coil 28 are key parts of a voice coil motorwhich applies a force to the actuator assembly 20 to rotate it about theactuator shaft 18. Also mounted to the housing 12 is a spindle motor 32.The spindle motor 32 includes a rotating portion called the spindle hub33. In FIG. 1, a single disk 34 is attached to the spindle hub 33. Inother disk drives a number of disks may be attached to the hub. Theinvention described herein is equally applicable to disk drives have anumber of disks attached to the hub of the spindle motor.

Also attached to the housing 12 is a ramp structure 36. Now looking atboth FIGS. 1 and 2, the ramp structure has two ramp portions 38. One ofthe ramp portions 38 is for the loading and unloading the transducerfrom the bottom surface of the disk and the other ramp portion is forloading and unloading a transducer from the top surface of the disk. Theramp structure 36 shown in FIG. 2 is fixed and a portion of each of theramp portions 38 of the ramp is positioned over the disk 34. It shouldbe noted that this invention could also be used on ramps that rotate inand out of a load/unload position.

FIG. 2 is an isometric view detailing the ramp structure 36. The rampstructure 36 includes the ramp portion 38 and the ramp support structure40. The support structure 40 has a first opening 41 and a second opening42 therein which facilitate mounting the ramp 36 to a pair ofcorresponding pegs on the housing 12. The ramp structure 36 includes aninclined surface 44 and a parking detent 46. A portion of the rampstructure 36 or the support structure 40 over which the transducer(shown in FIG. 3) passes may also be made readable by the transducer.For example, in one of the preferred embodiments described in detailbelow a piece of magnetized tape is applied to the side of the rampportion 38 nearest the transducer. In another embodiment, the rampportion 38 could be sputtered or otherwise coated with a magnetic layer.This sputtered ramp portion could then be magnetized. In either of theseembodiments, the ramp portion nearest the transducer would then be ableto read the ramp portion during loading or unloading of the actuator andpositional information and velocity information could be determineddirectly from the ramp itself.

FIG. 3 details the load beam 24 and a slider 26 attached to the end ofthe load beam 24. The slider 26 includes at least one transducer 50.Signal carrying wires 52 are attached to each transducer 50 in theslider 26 and are routed along the edge of the load beam 24. Alsoattached to the load beam 24 is ramp riding member 54. The ramp rider 54includes an extension 56 which actually contacts the surface of the rampportion 38 of the ramp structure. The extension 56 of the ramp rider 54includes a curved portion which fits within the parking detent 46 of theramp portion 38 (shown in FIG. 2).

Now turning to FIG. 4, the first of the preferred embodiments will beshown and described. FIG. 4 shows an actuator assembly 20 similar to theone shown in FIG. 1. The actuator assembly 20 shown in FIG. 4 includesthe slider 26 and the transducing head 50 at one end and the voice coil28 at the other end. Also shown in FIG. 4 is a servo system which can bethought of as having two sets of electronics--one for servoing on thedisk surface 70 which is on the left half of FIG. 4 and one for servoingacross the ramp structure 80 which is on the right half of FIG. 4.Common elements of both servo systems are a ZOH (Zero Order Hold) device67 and a voice coil motor driver 68. The ZOH device outputs a certainlevel of signal until the next input is fed to the device, like adigital to analog converter with hold capability. In other words, theoutput of the ZOH device 67 is a staircase function.

The ZOH device 67 takes the signal from one of the servo systems andoutputs a desired amount of current to the voice coil motor 28 until thenext signal is input to the ZOH device 67. The voice coil motor in turn,controls the transducer while in transducing relation with the disk 34and controls the slider and transducer as it is going up (unloading theheads) or down (loading the heads) the ramp portion 38 of the rampstructure 36. A switch 69 under control of a supervisor 71 switchesbetween the disk servoing electronics 70 and a ramp servoing electronics80. The supervisor 71 is a portion of a microcontroller, such as a partnumber TM5320M25 available from Texas Instruments.

Now looking more closely at the electronics schematically shown in FIG.4, the disk servo electronics 70 include an arm electronics module 72, aposition error signal generator 74 and an actuator seek and positioncontroller 76. The disk servo electronics are well known in the art ofdisk drives and, therefore, the discussion of their description andoperation will be abbreviated. In operation, the signal from the readportion of the transducer 50 travels across the signal carrying wires 52(shown in FIG. 3). The line 73 of FIG. 4 depicts the signal carryingwires between the transducer 50 to the arm electronics module 72. Thearm electronics essentially cleans up and amplifies the signal. Thesignal from the aim electronics module 72 is output to the positionerror signal generator which compares the positions as read to thedesired positions and outputs a position error signal to the actuatorseek and position controller 76. In the actuator seek and positioncontroller 76, velocities are calculated or estimated and compared tothe desired velocities. The actuator seek and position controller 76 isactually a multipurpose microprocessor so the velocities are calculatedalong with other calculations. The actuator seek and position controller76 outputs a signal to control the actuator so that it will more closelymatch the desired position or velocity at the next sample time. The diskservo electronics 70 will only be used when the transducer 50 is overthe disk 34 so during this time the switch 69 will be in the "b"position as shown in FIG. 4.

The ramp servo electronics 80 include a small button magnet 81 attachedto the actuator assembly 20, a Hall effect sensor 82 fixedly attached tothe housing 12 near the small button magnet 81, an adaptive velocitylinearizer 86, a source of desired ramp velocity 87, a summing junction88 and a digital velocity compensator 89.

In operation, the distance between the small button magnet 81 and theHall effect sensor 82 will increase when the transducer 50 is beingloaded on the disk 34 as it travels down the ramp portion 38. Whenunloading the transducer, the distance between the small button magnet81 and the Hall effect sensor 82 will decrease as the transducer travelsup the ramp portion 38. The relationship just described describes adrive where the Hall effect sensor is mounted on the outermost edge ofthe disk 34. This relationship would change if the Hall effect sensorwas mounted elsewhere. Most importantly, as the distance between theHall effect sensor 82 and the small button magnet 81 varies, the outputfrom the Hall effect sensor 82, depicted by lines 84, varies as aninverse like function of the distance.

Thus, the output from the Hall effect sensor 82 gives positionalinformation. Given certain sampling times, this discrete positionalinformation can be converted to velocity information. The adaptivevelocity linearizer 86 converts the analog signal from the Hall effectsensor 82 to a series of digital outputs. The adaptive velocitylinearizer 86 also converts the nonlinear output from the Hall effectsensor to a linear function. The output of the adaptive velocitylinearizer, which is the actual measured ramp velocity, is compared tothe desired ramp velocity at the summing junction/adder/comparator 88.The difference between the desired ramp velocity and the actual rampvelocity is the output of the summing junction/adder/comparator 88. Theoutput of the summing junction 88 is input to the digital velocitycompensator 89 which produces a control signal so the actuator will moreclosely match the desired ramp velocity. The ramp servo electronics 80will only be used when the transducer 50 is over the ramp portion 38, soduring this time the switch 69 will be in the "a" position as shown inFIG. 4.

FIG. 5 shows how the output voltage from the Hall effect sensor 82varies with respect to the distance x between the Hall effect sensor 82and the magnet 81 attached to the actuator assembly 20. Two curves arealso shown in FIG. 5. Curve 90 is at a first temperature and curve 92 isat a second temperature. FIG. 5 shows that the output voltage alsovaries with temperature and also shows graphically that the shape of thecurve does not change drastically in the presence of a temperaturechange. The curve merely shifts in response to a temperature change. Theshift occurs mainly because the change in temperature changes thestrength of the magnetic field. Thus for a given value of x the value ofthe output of the Hall effect sensor changes in the presence of atemperature change. Also worthy of note is that the slope at the pointcorresponding to the given value x or the differential on the firstcurve y=f(x) differs from the slope at the point on the second curvey=f(x) for the same given value of x. This is due to the fact that whenthe entire curve essentially shifts as a result of the temperaturechange, value of x remains the same so that the point slope taken at apoint y for a given value of x is essentially taken at a different pointon the shifted curve. The importance of this will be pointed out in thefollowing paragraphs.

Speaking generally, in order to have an effective method to determinethe velocity of the transducer and slider as it moves up or down theramp, one must have an accurate measure or an accurate estimate of thelocation of the slider or transducer on the ramp at a given time. If anaccurate measure or estimate of the position can be made at two sampletimes, then the velocity can be determined simply by subtracting thefirst position from the second position to determine the distancetraveled and dividing by the difference in the sample time.

Since the voltage output of the Hall effect sensor (y=f(x) on either thefirst curve 90 or the second curve 92 varies as a function oftemperature, it is difficult to correlate the output of the Hall effectsensor to actual positions on the ramp without additional information orwithout making an assumption. One way to estimate the initial positionsof the actuator arm with respect to the ramp is to make an assumptionabout the position of the curve of the output of the Hall effect sensory(x). One could assume a set of initial startup conditions which wouldinclude the typical temperature at which the drive will start up. Theoutput of the Hall effect sensor at several known positions would bemeasured shortly after a typical startup under a set of "typical"conditions. The values would then be stored in a nonvolatile memoryspace. This will work as long as the startup conditions remain constanteach time the disk drive is started. Problems occur when he startupconditions do not match the "typical" conditions assumed. When thisoccurs then the output of the Hall effect sensor 82 will not correlateto a specific distance between the small button magnet 81 and the Halleffect sensor 82.

Another method for correlating the location of the curve of voltageoutput from the Hall effect sensor y(x) verses the distance between thesmall button magnet 81 and the Hall effect sensor 82 has been developed.The method, in very simplistic terms, is to make an estimate of thepoint slope at y=f(x) for a known value of x. Since the point slopevaries depending on where the curve has shifted to for a given knownvalue of x, a person can accurately estimate the position of the curvey=f(x) if the point slope is known. Once the shifted position of thecurve y=f(x) is known, the output of the Hall effect sensor 82 can becorrelated so that the distance between the Hall effect sensor 82 andthe small button magnet 81 can be accurately estimated. The known valueof x that is used, x₀, is the location of Hall sensor 82 with respect tothe small button magnet 81 while the actuator has the slider with thetransducer therein parked on the ramp structure 36.

Now turning to FIGS. 6 and 7, we will begin discussing the method andmeans for estimating the location of the curve y=f(x)=the output of theHall effect sensor vs. the distance between the small button magnet 81and the Hall effect sensor 82. FIG. 6 shows one curve y which is theoutput of the Hall effect sensor 82 as a function of increasing distancefrom the small button magnet 81. This curve y shown in FIG. 6corresponds to any one of a number of curves that occur at a particulartemperature and include the curves 92 or 90 shown in FIG. 5. FIG. 6shows a value X₀ which is the distance between the button magnet 81 andthe Hall effect sensor 82 when the transducer is parked on the rampstructure 36. FIG. 6 illustrates that the distance x between the Halleffect sensor 82 and the button magnet 81 is very short compared to thetotal distance x over which the curve y operates. Therefore, only arelatively small portion of the curve y is used in a disk drive due tothe short stroke of the actuator assembly 20 (shown in FIG. 1). As aresult, one could reasonably assume that the function y=f(x) ispiecewise linear since the distance x due to the stroke of the actuatorassembly is small compared to the total distance x over which the curvey=f(x) operates. In a 1.8" disk drive, the total length of travelbetween the button magnet 81 and the Hall effect sensor 82 isapproximately 3 millimeters. In a 3.5" drive the total distance betweenx₀ and x max is about the same. Since the distance over which x variescovers a very small portion of the curve y=f(x), one can make theassumption that the function y=f(x) is essentially piecewise linear inthe affected region. Therefore, making this assumption, provides alogical starting point--namely that y(x) can be stated as a linearequation, which is as follows:

    y(x)=Γ(B,I) α(I)+β(I)x!                   EQ 1

where,

Γ(B,I)=an unknown nonlinear gain constant;

B=the magnetic field strength

I=the Hall effect sensor current

α(I)=a non-linear function of the current I

β(I)=a non-linear function of the current I

Now having assumed that the equation is linear, this can be written

    y(x)=Γ(B,I) α(I)!+Γ(B,I) β(I)x      EQ 2

From simple algebra, the slope term in an equation of the form y=mx+b ism. Thus, in Equation 2 the slope of the line is:

    Slope=Γ(B,I) β(I)                               EQ 3

A robust estimate of the slope is defined as β (beta hat) ##EQU1##

where y(x₁) is the average Hall effect voltage per integral revolutionmeasured when the actuator is track following on cylinder C, and x₁ isthe separation between the button magnet and the Hall device. Similarly,at a given cylinder C2, y(x₂) at distance x₂ is measured. Note that dueto the high precision used for servo writing, the difference x₂ -x₁ isknown with a high degree of accuracy. It is this accuracy that gives ussuch a superior estimation of the slope.

Now returning to Equation 1, an estimate α (alpha hat) and a ratio ηwill be derived. First, solve equation (1) for α(I)Γ(B,I) as shown inEquation (5) below and let x=x₀ =x₀ (nom)

    α(I)Γ(B,I)=y(x.sub.0)-β(I)Γ(B,I)x.sub.0 (nom) EQ 5

Next define the estimate of α, α (alpha hat), as the

    α=α(I)Γ(B,I)                             EQ 6

From Equation (2) we defined the estimate of β, β

    β=β(I)Γ(B,I)                               EQ 7

Substitution on equations (6) and (7) into (5) yields

    α=y(x.sub.0)-β x.sub.0 (nom)                    EQ 8

Now a ratio η, with α from equation 6 and β from Equation 7, can be setup. As you see in Equation (9) below, the unknown non-linear gainconstant Γ(B,I) cancels in the ratio η. ##EQU2## Since Γ(B,I) cancels,the ratio η remains fairly constant despite variations in the magneticfield B due to temperature, i.e., η is independent of Γ(B,I).

Substituting α and β into Equation 1, as α and β are defined inEquations (6) and (7) above, yields the following:

    y(x)=α+β x                                      EQ 10

From Equation (9),

    η·β=α                              EQ 11

Substituting for α in Equation (10) with the equality of Equation (11)yields:

    y(x)=η·β+β x                        EQ 12

Solving Equation (12) for β yields the following Equation (13): ##EQU3##

Equation 13 is important since it can be used to estimate β while theactuator is in park position after η has been estimated and stored innonvolatile memory. When the actuator is in the park position, thesuspension is placed on the ramp, at x₀ (nominal), to within an error ofless than plus or minus 0.005 millimeters, which is the manufacturingtolerance of the various drive components making up the actuator. Theratio η is determined from measuring the actual value of Γ(B,I) β(I) xand Γ(B,I) α(I) at two tracks on the disk. At manufacturing time, thedistances x₁ and x₂ can be determined and repeated very closely usingwhatever servo system is on the disk. Once η is measured, one can alwaysestimate the slope of the curve or β by taking a measurement of y(x₀(nominal)) while the actuator is in the park position since both x₀(nominal) and η are known. Before η is estimated for the first time, anominal or manufacturing value can be used initially. The estimate of βcan then be used to find the velocity of the head as it travels down theramp since the starting point x₀ nominal is known and the slope orvariation of the Hall sensor output (y₀) with respect to distance isalso known. This process is very robust to changes in temperature andallows the use of the disk drive under adverse temperature conditions.

From this, the velocity of the head going down the ramp can be derivedquite simply. The Hall effect sensor voltage y(x) is sampled at afrequency ##EQU4## Considering the nth and the (n-1)th samples, thenv_(n) the sampled velocity, can be written: ##EQU5##

The far right hand side of the above equation contains variables whichare either known or measured by the Hall effect sensor at the sampletimes on either side of (T_(s)). This last portion gives the velocityand also becomes the control equation for the system to control thevelocity of the head down the ramp.

FIGS. 8A and 8B show the steps necessary for operation of the rampcontrol system. Now turning to FIG. 8A, the details of calibrating theramp velocity control up for the very first time will be discussed. Thefirst step is to determine if this is the first time a particular diskdrive has ever been started. This is represented as decision box 120. Itshould be noted that this box represents the very first time the diskdrive is ever started rather than each time the disk drive is started.In other words, decision box 120 represents a one time event when thereis no data from the disk drive from previous times the drive has beenrun. Initially when the newly manufactured disk drive is started for thevery first time, there is no data from the drive itself on which to baseany estimates. In this instance, data which applies to each drive of asimilar design is used to place the head on the disk. Before theactuator and head go down the ramp, the voltage output of the Halleffect sensor 82 in the magnetic field of the small button magnet 81 ismeasured while the actuator is in the park position at the top of theramp, As shown in FIG. 7, f(x₀) is defined, for the sake of simplicity,as the distance between the Hall effect sensor 82 and the small buttonmagnet 81 while the actuator assembly 20 is in the park position. Thiscan be thought of as shifting the y-axis in FIG. 6 to its its newposition in FIG. 7. The result is that when x=y₀ you are actuallymeasuring the y intercept which is α in the equation y(x)=βx+α EQ 10!.This is represented by step 121 in FIG. 8A. Next, as represented by step122, a nominal or manufacturing value of η is assumed. The measuredvalue of y(x₀), the assumed η (nominal), and x₀ (nominal) are used toestimate β nominal, as indicated by step 123. X₀ (nominal) is theposition on the ramp within manufacturing tolerances. Therefore, βnominal is an estimate calculated from one measured variable and twoassumed values. The next step, 124, is to use the β nominal estimate tocontrol the slider as it moves down the ramp for the first time. Onsubsequent trips down the ramp, y(x₀) is measured for an estimate of α.The next step, 126, is to measure the voltage output value, y(x), fromthe Hall effect sensor at two track locations which are at a preciselyknown distance from one another. This is because the physical locationsof all concentric tracks in a hard disk drive are magnetically recordedon the disk surface at manufacturing time by a highly accurate externalpositioning system. Therefore, the distance between two arbitrary tracksis known to within a tolerance of a few microinches. The disk servosystem 70, such as an embedded servo system or dedicated servo system,is used to precisely locate and follow a given track to within a fewmicroinches. Thus the distance between two tracks are known with a highdegree of accuracy. With such an accurate "yardstick," one gets asuperior calibration. The disk servo system 70 is used to track followon the desired track for several complete revolutions and the output ofthe Hall effect sensor is averaged for the duration of the trackfollowing time. The known positions on the disk must be averaged over anintegral number of revolutions to cancel out any repeatable runout ofthe tracks on the disk. Once two values of y(x) are found, β can befound using Equation 4 above. Once α and β are known, the ratio of η canthen be calculated for the system, as shown in step 128. This value isstored, step 130, and updated on occasion. Decision box 132 representsone way to determine recalibration based on an elapsed desired time, t.There are, of course, a number of different criteria that can be used asselection criteria for recalibrating the system. Recalibration of thevalue η requires a repeat of Steps 126, 127, 128, and 130. In Step 130,the value of y(x₀) used is the last measured value of the Hall effectsensor when the head was positioned at x₀ in the parking detent on theramp.

Now turning to FIG. 8B, the operation and advantages of the inventionwill be discussed. After the initial calibration, the number for η willbe known and can be called from non-volatile memory as depicted by box134. While the actuator assembly 20 is in a parked position at x₀, thevoltage output from the Hall effect sensor 82 is measured therebyyielding y(x₀) as depicted by box 136. The distance x₀ is also known, asshown by box 138. The next step is to use the measured y(x₀), the knownx₀, and the calculated η to derive an initial estimate at an unknowntemperature of the slope of the curve or β using the formula shown inbox 140. Once the slope of the curve and the starting point x₀ is known,an accurate estimate of distance as represented by the voltage outputfrom the Hall effect sensor can be determined. Of course, position couldbe used to control the movement of the actuator down the ramp; however,in the preferred embodiment velocity is estimated using the equationshown in box 142. The velocity estimate is then compared to a desiredvelocity and the difference, if any, is used to control the current tothe coil of the voice coil motor in an attempt to make the estimatedvelocity match the desired velocity at the next sample time, as shown atbox 144. The slope β can be updated while the hard disk drive is innormal operation by the application of EQ 4.

FIG. 9 shows a second preferred embodiment of the invention. Thispreferred embodiment has many of the same elements, however, an opticalwhite target 100 and an optical source and detector 102 have beensubstituted for the Hall effect sensor and the magnet. The opticaltarget is attached to the actuator assembly 20. This preferredembodiment would work by having the distance calibrated directly by anoptical source and detector 102 detecting the intensity of light fromthe optical target 100 attached to the actuator assembly 30. The shapeof te optical target 100 can be changed to provide different intensitiesof light. The shape can be changed to produce various functionalrelationships between distance and light intensity. The optical sourceand detector output can be converted from positional information tovelocity information as the transducer passes over the ramp portion 38.This actual, calibrated velocity information is compared to the desiredvelocity from the source of desired ramp velocities 87 in thesummer/comparator/adder 88. The output which is the difference betweenthe actual and desired velocity is sent to the digital velocitycompensator 89 where a signal is output to control actuator so that itapproaches the desired velocity at the next sample time. As before, thesupervisor 71 switches between the ramp servo electronics 80 and thedisk servo electronics 70. The disk servo electronics 70 work inprecisely the same way as in the preferred embodiment with the Halleffect sensor discussed above.

In another embodiment, a light source and one-half of an optical gridcan be attached to the housing. This would substitute for the Halleffect sensor or the detector and sensor 102. The other half of anoptical grid could be attached to an actuator so that an optical grid isformed. Movement of the actuator causes movement within the grid whichin turn causes the intensity of a light source to vary sinusoidally. Thevariations in intensity can be transformed to a signal which varies. Thevariations can be counted to give a direct measure of position andvelocity with respect to time. With two light sources, the sources canbe positioned so that the signals produced are 90° out of phase from oneanother. With two signals 90° out of phase from one another, thedirection of the moving half of an optical grid can be determined.

It is further contemplated that in a disk drive the optical electronicscould be used to servo on the disk as well as on the ramp. This wouldgenerally require a very fine grid 100.

FIG. 10 shows still another embodiment, in which a capacitance probe hasbeen substituted for the Hall effect sensor 82 or the grid and opticalreader shown in FIG. 9. In this particular embodiment, the distancemeasure is made on the basis of capacitance between two plates. Like theHall effect sensor, the measured capacitance between two plates willvary as a function of distance. The difference in the measuredcapacitance can be used to determine the distance between the actuatorand the probe. In this embodiment, a capacitance probe 104 is attachedto the housing 12. A plate 106 is attached to the actuator assembly 20.Of course, as well understood by those skilled in the art, the probe 104and the plate 106 could very easily be switched and this embodimentwould be equally effective.

The difference in capacitance would be used to determine velocity of thehead as it moves down the ramp. The capacitance probe 104 output can beconverted from positional information to velocity information as thetransducer passes over the ramp portion 38. This actual, calibratedvelocity information is compared to the desired velocity from the sourceof desired ramp velocities 87 in the summer/comparator/adder 88. Theoutput which is the difference between the actual and desired velocityis sent to the digital velocity compensator 89 where a signal is outputto control actuator so that it approaches the desired velocity at thenext sample time. As before, the supervisor 71 switches between the rampservo electronics 80 and the disk servo electronics 70. The disk servoelectronics 70 work in precisely the same way as in the preferredembodiment with the Hall effect sensor discussed above.

With such a system there would be two sets of servo electronics muchlike that shown in the previous embodiment. An intelligent controllersupervisor 71, which is generally a portion of a microcontroller,switches between the ramp servo system 80 and the disk servo system 70associated with the disk. The ramp servo system 80 would operate whilethe head is travelling over the ramp structure and the disk servo system70 would operate while the head is travelling over the disk 34.

Now referring to FIGS. 14A, 14B and 15, a preferred arrangement for acapacitive sensor will be discussed. As shown in FIGS. 14A and 14B, acapacitive plate 200 is physically attached to the housing 12 of thedisk drive 10. The capacitive plate 200 is electrically isolated fromthecasting forming the housing so that a charge may be placed on thecapacitive plate 200 rather than on the entire housing 12. Thecapacitive plate, which acts as one plate of a capacitor, is located sothat it fits between the arms 23 of the actuator assembly 20 with adesired amount of clearance. The arms 23 typically are flat platesbetween the axis of rotation and the area where a suspension is attachedto the arm. The arms 23 of the actuator assembly 20 are grounded via aline in the flex cable (not shown). Therefore, the arms 23 act asgrounded plate of a capacitor. As the actuator assembly 23 rotates tomove the transducer down the ramp 36 (not shown), the area of the arm 23in close proximity to the capacitive plate 200 varies which in turnvaries the capacitance.

Now looking to FIG. 15, a wire 202 is attached between the capacitiveplate 200 which is part of a circuit 204, shown in FIG. 15. The circuit204 is actually an RC circuit which acts as a variable oscillator. Thecircuit 204 includes the variable capacitor comprised of the capacitiveplate 200 and the arms 23 of the actuator assembly 20, a Schmidt trigger206, and a resistor 208 attached to a voltage supply 210. Of course,some of the components of the circuit 204, with the exception of thecapacitive plate 200 and the arms 23, can be formed into a chip or madewith discrete components. One example of such a chip is available fromTexas Instruments as part number SN74ASC2502. The capacitive plate 200and the arms 23 of the actuator assembly 20 are the capacitor in thecircuit. The capacitor formed is variable since the area of thecapacitive plate 200 in close proximity to the arms varies. As is wellknown in the art, the capacitance between two plates varies with thearea in close proximity. The capacitance between the plates of acapacitor is also inversely proportional to the distance between theplates of the capacitor. Therefore, the clearance between the arms 23and the capacitive plate 200 is selected to yield a desired capacitance.

The operation of the capacitive sensor will now be discussed with theaid of FIG. 15 and one more basic, well known principle. The principleis that an RC circuit has a time constant which means, in simple terms,that a capacitor can carry a certain level of charge and that the timeit will take to build up to this level of charge from a resistorconnected to a voltage source will vary dependent on the amount ofresistance and the amount of capacitance in the RC circuit. The resistor208, in the RC circuit formed, is held constant. The capacitor, formedby the capacitive plate 200 and the arms 23 of the actuator assembly 20,vanes as the actuator assembly 20 rotates. Since the capacitance varies,the time constant of the circuit will also vary. The total capacitanceof the RC circuit formed includes the variable capacitance between theplate 200 and arms 23, described above; the stray capacitance betweenthe plate 200 and the housing casting; the lead capacitance to ground;and the circuit input capacitance

In operation, a variable frequency oscillator is formed by the circuit204. The frequency at which the circuit oscillates is dependent on theposition of the actuator assembly 20 with respect to the capacitiveplate 200 because the position of the arms 23 with respect to the plate200 varies the capacitance in the circuit 204. The Schmidt trigger 206shorts the capacitance to ground when the voltage at the Schmidt trigger206 in the circuit 204 reaches a certain level. After the capacitance isshorted to ground, the capacitor begins to recharge up to the certainlevel where the Schmidt trigger again shorts the capacitance to ground.This process repeats over and over. The time necessary for the voltageat the Schmidt trigger to reach the level where the Schmidt trigger 206shorts the capacitance to ground varies with the capacitance or theposition of the actuator arms 23 with respect to the plate 200. Thus,the frequency of the oscillator circuit formed varies as a function ofthe position of the arms 23 with respect to the plate 200 and can beused to locate the position of the transducer 50 with respect to theramp structure 36.

In practice, the frequency change between the position of the actuatorassembly 20 where the transducer 50 is in the parking detent 46 and theend of the ramp structure is the important measure. Although thecapacitance will change as a function of temperature, the variation inthe frequency range through which the RC circuit passes as the actuatorassembly 20 moves the transducer over the ramp 36 does not alterdrastically. Therefore, when initially starting the disk drive with thetransducer 50 in the parking detent 46, the frequency is measured. Thefrequency at the end of the ramp 36 is also measured. The difference inthe frequencies, or the range of the frequency, is stored in nonvolatilememory. When the transducer 50 is to be moved down the ramp 36 afterinitial starting the disk drive, the frequency while in the parkposition is recorded and the range figure is added to the initialfrequency. From this, one can always determine the position of thetransducer 50 with respect to the ramp 36. The positional informationobtained can then be used to determine the velocity over a sample timeand this can be compare to a desired velocity and correction can takeplace in a closed loop process. Advantageously, the frequency changedown the ramp will be in roughly a one to one correspondence to theposition of the transducer 50 with respect to the ramp 36. Once thetransducer 50 has passed over the ramp and onto the disk, the RC circuit204 may be disabled to prevent any interference with the read and writeoperations in the disk storage device.

FIG. 11 shows yet another preferred embodiment. In this embodiment, atransducer 50 in the slider 26 is used as the sensor for determiningposition on the ramp as well as used as the sensor for determining theposition on the disk. In this embodiment, the ramp portion 38 includesan additional ledge 39 which is designed to be near the transducer 50 inthe slider 26 while the actuator assembly 20 is moving the slider 26over the ramp portion 38. The ledge 39 could be designed to be part ofthe ramp support structure 40. The ledge 39 includes a portion which canbe magnetized. In FIG. 11 this is shown as a layer on the surface 39'near the transducer 50. The surface 39' of the ledge 39 is magnetizedwith a servo pattern readable by the transducer 50. The surface 39'could be an applied magnetic strip, a magnetic layer directly sputteredon the ledge 39, a layer of material containing iron oxide particles, orany other known layer of magnetizable material. It should be pointed outthat the entire ramp structure 36 (shown in FIG. 2) or certain portionsin addition to the ledge 39 could also be covered with a magnetizablesurface.

FIG. 12 shows a variation of the embodiment of the invention shown inFIG. 11. In this variation, the ledge 39 is provided with a currentcarrying wire 43. The wire 43 in the preferred embodiment is part of aflexible circuit 172 which has a shape corresponding to the ledge 39 ofthe ramp portion 38. The flexible circuit 172 is attached to the surfaceof the ledge 39. The wire 43 could also be embedded into the ledge 39.It should be noted that the wire 43 preferably is at an angle withrespect to the edge of the ledge 39 and to the path of travel of thetransducer 50. The angle shown is approximately 45 degrees with respectto the path of travel of the transducer 50 (shown in greater detail inFIG. 13). At this angle a magnetic field around the wire includes acomponent which is in the same direction as the transitions on the disk34 (shown in FIG. 1). Of course, this angle is not absolutely necessaryand the angle could be varied to accomplish the same purpose asdescribed above. It should also be noted that in the case of verticalmagnetic recording the component of the magnetic field of the wire 43 ofinterest would be normal to the plane of the disk 34 and the angle ofthe wire 43 with respect to the path of travel of the transducer 50would not be critical

The current carrying wire is attached to a current generator 150. Thecurrent generator is switched on when the actuator assembly 20 (shown inFIG. 1) moves the transducer 50 over the ledge 39. The current generatoris switched off when the transducer 50 is in transducing relationshipwith the disk 34. Switching the current generator 150 off prevents noiseor interference with the transducer 50 while it is positioned to readdata and also reduces the total overall power consumption of the drive.

FIG. 12 also includes the electronic circuit 152 for reading themagnetic transitions produced by the current carrying wire 43 and themagnetic transitions on the disk 34. The electronic circuit includes anarm electronics module 154 and a read channel 156. Both of thesecircuits are well known in the art and therefore will be described verygenerally. The arm electronics module amplifies and cleans up the signalread from the disk. The read channel either decodes the read signal andships data from the disk to a computer system or encodes the data from acomputer system and places on the disk. The electronic circuit 152 alsoincludes a low frequency demodulator 158 and a regular or higherfrequency demodulator 160. A switch 162 switches between the lowfrequency demodulator 158 and the higher frequency demodulator 160. Thecircuit also includes a band pass filter 164 and a radial positionindicator 166 and an AND GATE 168. The electronic circuit 152 isattached to a transducer 50 which reads a disk 34 also shown in FIG. 12.The disk 34 includes a ramp structure 36 positioned at the outerperiphery of the disk 34. The disk 34 includes an area 170 in the outertrack or tracks of the magnetized portion of the disk 34 that includestransitions that occur at a constant frequency. The area of the diskthat is written at constant frequency is near the outer periphery of thedisk which is also near the ramp structure 36.

In operation, the electronic circuit 152 for reading magnetictransitions operates as described in this paragraph. The arm electronicsmodule 154 and the read channel 156 operate in a conventional manner.When the transducer 50 passes over the constant frequency portion 170 ofthe disk, the signal produced is of constant frequency. The output fromthe arm electronics module 154 is also a constant frequency signal. Theconstant frequency signal from the arm electronics module 154 is inputto the bandpass filter 164 which produces an output when the frequencyof the signal input matches the frequency of a signal produced fromreading the constant frequency pattern 170 on the disk 34. A radialposition detector 166 outputs a signal when the actuator arm (not shown)has the transducer 50 positioned over the outer tracks of the disk. Whenthere is an output from both the band pass filter 164 and the radialposition detector 166 the AND GATE produces an output signal. The radialposition detector 166 assures that the output from the AND GATE does notoccur if by chance a constant frequency signal is produced while thetransducer 50 is positioned over another area other than the constantfrequency portion 170 of the disk 34.

The output of the AND GATE 168 controls the current generator 150 andthe switch 162 between the low frequency demodulator 158 and the higherfrequency demodulator 160. The AND GATE output can be thought of as atrigger signal for switching between reading the transitions produced bythe current carrying wire 43 positioned on the ledge 39 of the ramp andthe transitions on the disk 34. Basically the output of the AND GATE 168causes the current generator 150 to turn on or off and causes the switch162 to switch between the low frequency demodulator 158 and the higherfrequency demodulator 160. When the transducer is over the disk andreading or writing data to or from the disk, the power to the currentcarrying wire is in the off position and the switch 162 is in a positionwhere the higher frequency demodulator 160 is being used to demodulatethe data. When the transducer is traveling over the ledge 39 of theramp, the current generator 150 is on so that current is produced in thecurrent carrying wire 43 and the switch is in a position so that the lowfrequency demodulator 158 is being used to read the transitions producedby the wire 43.

When the transducer 50 passes over the constant frequency area 170 ofthe disk 34, it triggers the switching in the electronic circuit 152.For example, while reading data, the signal produced by the transducer50 is demodulated with the high frequency demodulator 160 and thecurrent generator 150 is off. The actuator moves the transducer towardthe ramp structure 36 and passes into the constant frequency region 170of the disk. The constant frequency region 170 causes a constantfrequency signal and an output from the band pass filter 164 which isinput to the AND GATE 168. The radial position indicator 166 outputs asignal when the actuator has the transducer positioned near the outerperiphery of the disk which corresponds to the constant frequencyportion 170 of the disk 34. The output of the radial position indicator166 is input to the AND GATE as well. In response to inputs from theradial position indicator 166 and the band pass filter 164, the AND GATE168 outputs a signal to the current generator 150 and the switch 162. Inresponse to the AND GATE signal, the current generator turns on and theswitch moves from a position where the signal from the transducer ispassed through the high frequency demodulator 160 to a position wherethe signal from the transducer is passed through the lower frequencydemodulator 158. Therefore, as the transducer 50 goes up the ramp towarda park position, there is current in the wire 43 and the lower frequencydemodulator 158 is being used to read the transitions produced.

When in the park position, the current generator 150 is on and producingcurrent in the wire 43. The switch 162 is positioned so that the lowerfrequency demodulator 158 is ready to read transitions. As thetransducer is moved down the ramp and over the ledge 39, the lowfrequency demodulator 158 demodulates the signal. The transitionscounted in the demodulated signal give a direct indication of theposition of the transducer with respect to the ledge 39 and the ramp.This positional information can be used to calculate the velocity of thetransducer 50 as it travels down the ramp. The calculated velocity canbe compared to a desired velocity for a particular position and thedifference can be used as feedback to control the current to the voicecoil of the voice coil motor of the actuator to produce the desiredvelocity at the next position down the ramp. After the transducer 50travels down the ramp, it passes over the constant frequency area 170 ofthe disk 34. The constant frequency area causes a signal with a constantfrequency which is input to the band pass filter 164. The band passfilter 164 outputs a signal which is input to the AND GATE 168. At thesame time the radial position indicator 166 indicates that thetransducer 50 is positioned over the constant frequency area 170 at theouter periphery of the disk 34 and inputs a signal to the AND GATE 168.As a result, the AND GATE 168 outputs a signal which is input to boththe current generator 150 and the switch 162. The current generator 150is turned off upon receiving the signal from the AND GATE 168 and theswitch 162 is positioned to the higher frequency demodulator 158 toallow demodulation of the data when read. Thus, as can be seen, theconstant frequency area of the disk 170 produces a constant frequencysignal which produces an output from the band pass filter 164. Theradial position indicator assures that the transducer 50 is reading onthe constant frequency area and causes the AND GATE 168 to produce atrigger signal that switches the current generator on or off andswitches to the other of two demodulators. The state of the currentgenerator 150 and the switch 162 depends if data is going read from thedisk or if transitions are going to be read from the current carryingwire 43.

It should be noted that the current generator 150 doesn't have to be aseparate device but could be a portion of the power supplied to the diskdrive. It should also be noted that when the system has been powereddown, the disk drive will position the slider 26 and the transducer 50within it in a park position in the parking detent 46 atop the rampstructure 36. Therefore, when power is first supplied to the disk drive,the current generator 150 should be turned on and the switch 162 shouldbe positioned to use the low frequency demodulator 158.

This alternative embodiment has the advantage of not requiring amagnetized surface 39' or requiring the writing of a servo pattern ontothe surface 39'. In addition, particle generation, if any, will beminimized since no particles will be generated from the magnetizablesurface 39' being scraped off. In addition, the current to the currentcarrying wire 43 can be shut off while the actuator assembly 20 has theslider 26 and transducer 50 positioned over the disk. This minimizes thepossibility of having stray magnetic fields which might be detrimentalto the data stored on the disk. Of course, the current level in the wire43 would be designed to be low enough so that the magnetic fieldproduced by the current would not effect the magnetic field on a disk ifthe data was stored on the disk magnetically. Turning off the current inwire 43 would thus be an optional form of insurance.

As shown in FIG. 12, the wire 43 in this alternate solution would bepositioned so that the wire would cross the line of travel of thetransducer as the actuator assembly 20 moves the slider 26 which carriesthe transducer 50 over the ramp portion 38 of the ramp support structure40. Advantageously, the crossing pattern would produce magnetic fieldswhich are opposite each other so that the opposite fields could becounted while the actuator moves the transducer down the ramp.

The current in wire 43 may be an alternating current of moderatefrequency. This produces a significantly higher frequency than thatproduced by the lead passing over the wire sections with the oppositecurrent direction. When an alternating current is used that issignificantly higher frequency than the wire crossing frequency, thehigher frequency is amplitude modulated. If the amplitude is rectifiedand filtered, a high amplitude will be found when crossing each wire andzero voltage will occur between each wire. The higher frequency issignificant for systems containing coupling capacitors that were notintended to pass very low frequencies, and also for inductive read headsthat generate very low voltages at the very low frequencies of the wirecrossing rate that is generated when direct current is used.

The wire 43 would not provide the same amount of positional accuracy asa servo pattern on a disk. Such accuracy is not needed to get a measureof velocity over the ramp portion 38. As a result, a servo pattern, ifwritten to a surface 39' of magnetic material could also be less precisewhen compared to the servo pattern on the disk surface. The servopattern on the ramp would be different from the servo pattern on thedisk to provide for a way to easily differentiate between the two servopatterns.

Now turning to FIG. 13, a top view of a portion of a flexible circuit172 which fits on the ledge 39 of the ramp is shown. The currentcarrying wire 43 is within the flexible circuit 172. The currentcarrying wire is formed as a continuous path and crosses at an anglewith respect to the edge of the flexible cable 172 as is mentionedabove. In FIG. 13, the magnetic field produced by the current carryingwire 43 is shown as a vector H. A transducer 50 reads fields along aline perpendicular to the centerline of the ledge 39 or the edge of theledge 39. A representative line 173 alone which the transducer 50 ismost likely to read transitions has been added to FIG. 13. The line 173makes an angle theta with respect to the current carrying wire 43. Thus,the component of the magnetic field that will be read by the transducer50 is equal to:

    Hcos(π/2-θ)

which can be rewritten in expanded form as:

    H cos π/2 cos θ+sin π/2 sin θ!

since sin π/2=1 and since the cos π/2=0 then the expression can bestated as:

    H sin θ

Thus it can be seen that by increasing the angle theta that the currentcarrying wires 43 make with respect to line 173, that the readablecomponent of the vector H will become larger.

It should be noted that many other types of sensors could be used toperform this invention such as lasers.

The present invention and the best modes for practicing it have beendescribed. It is to be understood that the foregoing descriptions areillustrative only and that other means and techniques can be employedwithout departing from the full scope of the invention as described inthe appended claims.

What we claim is:
 1. A disk drive comprising:a housing; at least onedisk rotatably attached to said housing; an actuator assemblyincluding:an actuator; a transducer attached to said actuator, saidtransducer positionable to a transducing relationship with said disk;and means for moving said actuator to move said transducer from oneposition with respect to a disk to another position with respect to saiddisk; a ramp attached to said housing and oriented so that a portion ofthe actuator passes over the ramp to load or unload the transducer; anda sensor means associated with said actuator for directly sewing aplurality of positions of said transducer as the actuator passes overthe ramp without monitoring voltages in said means for moving saidactuator.
 2. The disk drive of claim 1 wherein said sensor meansassociated with said actuator includes a Hall effect sensor.
 3. The diskdrive of claim 1 wherein said sensor means associated with said actuatorincludes an optical sensor.
 4. The disk drive of claim 1 wherein saidsensor means associated with said actuator includes;a capacitance probeattached to one of said housing and said actuator; and a plate locatedon the other of said housing and said actuator so the capacitancebetween said capacitance probe and said plate varies as a function ofthe distance between said capacitance probe and said plate as saidactuator passes over said ramp.
 5. The disk drive of claim 1 whereinsaid sensor means associated with said actuator includes the transducerwhile the actuator is positioned on said ramp.
 6. The disk drive ofclaim 1 further comprising a servo system which includes;a first servomeans for positioning the transducer with respect to said disk; and asecond servo means for positioning the transducer with respect to saidramp.
 7. The disk drive of claim 6 further comprising a controller forswitching between said first servo means and said second servo means. 8.The disk drive of claim 1 wherein the sensor associated with saidactuator includes a variable capacitance capacitor of aresistor-capacitor (RC) circuit, said variable capacitance capacitorbeing arranged so the capacitance thereof varies as said actuator passesover said ramp.
 9. The disk drive of claim 1 wherein the sensor meansassociated with said actuator includes a current source and a variablecapacitance capacitor arranged so the capacitance thereof varies as saidactuator passes over said ramp.
 10. A disk drive comprising:a housing;at least one disk rotatably attached to said housing; an actuatorincluding:a transducer attached to said actuator, said transducerpositionable to a transducing relationship with said disk; and means formoving said actuator to move said transducer from one position withrespect to a disk to another position with respect to said disk; a rampattached to said housing and oriented so that a portion of the actuatorpasses over the ramp to load or unload the transducer; and a sensorassociated with said actuator for sensing the position of saidtransducer as the actuator passes over the ramp, wherein the sensorassociated with said actuator includes part of a resistor-capacitor (RC)circuit, and wherein the actuator further includes at least one arm andwherein the housing includes a capacitive plate attached to saidhousing, said at least one arm and said capacitive plate forming avariable capacitance capacitor in said RC circuit.